Most fiber structures are manufactured through extrusion processes where fibers are extruded from a die. For some types of fiber structures it is beneficial to use a drawing process, where a fiber preform is drawn into a fiber. Preform drawing can be used to produce fiber-like structures such as Polymer Optical Fibers (POFs), Micro-structured fibers, photonic crystal fibers, and Capillary Fibers.
One significant problem when drawing fibers from a preform is that diameter fluctuations can be caused by chaotic thermal air currents in the neck-down region of a preform. Indeed, thermal air currents can cause diameter variations of plus or minus 10% or more. Hayden Reeve, Ann Mescher, and Ashley Emery of the University of Washington (UW) Seattle have published papers concerning this problem. According to Hayden Reeve and Ann Mescher “the contribution of natural convection in glass optical fiber drawing has generally been neglected, natural convection accounts for approximately 30% of the heating during POF drawing in the absence of forced convection. Hayden M. Reeve & Ann M. Mescher, Effect of Unsteady Natural Convection on the Diameter of Drawn Polymer Optical Fiber, OPTICS EXPRESS 11-15, 1771 (2003). Furthermore, “unsteady natural convection during POF drawing can cause large (±15 μm) variations in the fiber diameter.” Id. In their papers they discussed methods of reducing the diameter fluctuations caused by the thermal air currents. One method that they found to work out the best is to feed the preform into the oven through a heated iris. The heating of the iris is designed to reduce the strength of the thermal currents by reducing the temperature differences between the preform and the surrounding air that drives these currents.
Heating of the iris, however, is often impractical to implement in high speed production systems since it requires tight temperature control of the iris and the oven. It also minimizes but does not eliminate the driving force behind these thermal currents oscillations.
Typically, when drawing plastic structures, the environment surrounding the preform is ignored. However, as demand for higher quality fibers increases, environmental effects on the diameter of drawn fibers should be addressed.